1. Field of the Invention
Aspects of the present invention relate to an optical recording and/or reproducing apparatus, and more particularly, to an optical pickup apparatus which can efficiently prevent degradation of a tracking signal due to an adjacent layer during reproduction of data from an optical disc having a plurality of recording layers.
2. Description of the Related Art
Optical discs are storage media for recording and reproducing data such as sounds, images, or documents by forming a large number of pits in a surface of an optical disc in order to change the reflection direction of a laser beam irradiated on the optical disc. Such optical discs typically include CDs (compact discs) and DVDs (digital versatile discs). Recently, high density recording media with a high recording capacity have been actively developed as a next generation of optical discs. The high density recording media typically include Blu-ray discs (BDs) and high definition DVDs (HD DVDs).
Data is recorded and/or reproduced to and/or from an optical disc using an optical recording and/or reproducing apparatus that uses an objective lens having a predetermined NA (numerical aperture) and a laser beam having a predetermined wavelength. The magnitude of the NA and the wavelength depend on the amount of information to be stored. That is, as the capacity of the optical disc increases, a light source of a shorter wavelength and/or an objective lens of a higher NA is used. For example, a laser beam of a 780 nm wavelength and an objective lens of 0.45 NA are used for a CD. A laser beam of a 650 nm wavelength and an objective lens of 0.6 NA are used for a DVD, which stores more data than a CD. A laser beam of a 405 nm wavelength and an objective lens of 0.85 NA are used for a BD, which stores more data than a DVD.
In other words, the recording capacity of the optical recording and/or reproducing apparatus to record and/or reproduce information to and/or from an optical disc using a light spot obtained by focusing a laser beam using an objective lens is in inverse proportion to the size of the focused light spot. The size S of a focused light spot is determined by the wavelength λ of a laser beam and the NA of an objective lens as shown by Equation 1.S ∝ K·λ/NA   [Equation 1]
(k is a constant which is based on an optical system and typically has a value between 1 and 2).
Thus, in order to increase the density of an optical disc, the size S of a light spot formed on the optical disc needs to be reduced. To reduce the size S of the light spot, as shown by Equation 1, the wavelength λ of a laser beam needs to be decreased and/or the NA needs to be increased.
However, expensive parts have to be used to decrease the wavelength λ of the laser beam. Also, when the NA of the objective lens is increased, the depth is decreased by a magnitude corresponding to the square of the NA. As a result, coma aberration is increased by a magnitude corresponding to the cube of the NA. Thus, there is a limit in increasing the density of an optical disc by reducing the size S of a light spot in the above two methods.
Although the DVD and BD have higher recording capacities compared to conventional media, due to a continuous need for an increase in the capacity of an optical disc, a multi-layer structure provided with a plurality of recording layers has been proposed. Accordingly, an optical disc having a plurality of multiple recording layers has been proposed, in which a single side or both sides of the optical disc have two or more recording layers. This multi-layer optical disc has a higher recording capacity than an optical disc having a single recording layer.
Meanwhile, a differential push-pull (DPP) method to correct an offset of a push-pull signal generated during the reproduction of an eccentric optical disc is generally used as a tracking method for a recordable optical disc. Typically, in the DPP method, a light beam is split into three components using a grating. The three components include, a 0 order light component (a main light component) and ±1 order light components (sub-light components). Regarding the relative magnitudes of the split light components, the light component ratio −1 order: 0 order: +1 order should be not less than 1:10:1 in consideration of light use efficiency, in other words, the 0 order main light component should have at least ten times the magnitude of the magnitudes of the ±1 sub-light components.
When the DPP method is used to detect a tracking error signal in a multilayer optical disc having a plurality of recording layers, for example, a dual layer optical disc having two recording layers, the tracking error signal is degraded because the 0 order main light component reflected from an adjacent layer overlaps with the ±1 order sub-light components reflected from a target layer. Since the difference between the magnitudes of the 0 order main light component reflected from the target layer and the 0 order main light component reflected from the adjacent layer is very large, the 0 order main light component of the adjacent layer does not affect a reproduction signal. However, the ±1 order sub-light components reflected from the target layer and the 0 order main light component reflected from the adjacent layer do not have a large difference in magnitude. Thus, the 0 order main light component of the adjacent layer substantially affects a differential signal (a sub-tracking signal (SPP)) used to detect the tracking error signal in the DPP method.
Even when the main light component reflected from the adjacent layer is input to a sub-detector, this does not necessarily affect cross-talk between the recording layers on an optical disc. If a light receiving magnification ratio condition satisfies Equation 2, the input of the main light component from the adjacent layer to the sub-detector does not cause any problem.
                                          S            pd                                M            2                          ≤        25                            [                  Equation          ⁢                                          ⁢          2                ]            
(Spd: the size of a main detector, M: a light receiving magnification ratio)
However, when the light receiving magnification ratio M is increased to satisfy the above condition, the size of a pickup increases, making it difficult to design a slim and compact pickup. Also, the increase of the light receiving magnification ratio degrades the adjustment and reliability characteristics of a detector. Thus, it is difficult to satisfy the above condition without causing practical problems.
Thus, to address the above problems, U.S. Patent Publication No. US 2005/0161579 A1 discloses a method of blocking a main light component reflected from the adjacent layer from being received by first and second sub-photodetectors using a polarization member (i.e., a polarization hologram) in order to diffract the main light component reflected from the adjacent layer to areas other than the area of a detector. In this publication, an optical pickup has a structure such that a polarization of the light proceeding from a light source to an objective lens and a polarization of the light reflected from an optical disc are orthogonal to each other. The polarization hologram is used only diffract the polarization of the light reflected from the optical disc.
FIG. 1 illustrates an example of a hologram pattern 253 of a polarization hologram 25 used in the above-described optical pickup. As shown in FIG. 1, since the polarization hologram 25 that blocks a signal light cannot be formed to have a large pattern, when the center of the polarization hologram 25 and the optical axis of an objective lens do not accurately match due to an assembly error, part of the main light component reflected from the adjacent layer is transmitted to the first and second sub-photodetectors, affecting the quality of a tracking error signal. Also, since P and S waves are used, a blocking effect is reduced with respect to an attachment angle error of the polarization hologram 25.
Nevertheless, a more serious problem is that the polarization hologram 25 not only blocks the main light component reflected from the adjacent layer from being input to the sub-detector but also blocks the main light component (a signal component) reflected from the target (i.e., instant) reproduction layer from being input to the photodetector. That is, the main light component reflected from the target layer is input to the main photodetector to generate an RF signal. Since part of the main light component is blocked by the polarization hologram, the amplitude of a signal to be detected decreases so that a signal characteristic, that is, a jitter characteristic, is degraded. Generally, the profile of an input light of a light receiving portion is of a Gaussian type, i.e., a bell curve. However, the polarization hologram 25 blocks the center area of the Gaussian profile, that is, a portion where the amplitude of a signal is maximum. Thus, the polarization hologram 25 seriously degrades the RF signal characteristics.
When the surface area of the polarization hologram 25 is reduced in order to reduce degradation of RF signal characteristic, it becomes difficult to achieve the original goal to suppress the main light component reflected from the adjacent layer so as not to be incident to the sub-detector. Moreover, if the light receiving magnification ratio is small, the blocking surface area of the polarization hologram 25 should be larger, thereby reducing the size of the RF signal and degrading the quality of the RF signal.
To address the above problem, U.S. Publication No. US 2005/0161579 A1 discloses a sub-photodetector that is additionally provided to compensate for the signal characteristic degradation by separately detecting diffractive lights which are diffracted to reach an area separated from the photodetector. However, since the signal used for the compensation, that is, the light signal which is diffracted to reach the separate area, already includes cross-talk noise, the signal is hardly useful for appropriately compensating for the RF signal.
Thus, a practical solution to improve the degradation of the RF signal characteristic is to finely adjust the polarization hologram to find a point where the effect by the adjacent layer is minimized and simultaneously the original signal from the target reproduction layer is maximized. However, this solution requires more parts to make the fine adjustment, increasing costs and manufacturing time.